The present invention relates to electronic circuits, and more particularly, to techniques for testing the AC transfer characteristic of a high-speed analog circuit using a built-in test circuit.
The transfer function of an amplifier is typically tested using external test equipment that is outside the integrated circuit the amplifier is fabricated in. The external test equipment sweeps the frequency of an attenuated sinusoidal test input waveform to the amplifier to generate an output sinusoidal waveform. The output sinusoidal waveform of the amplifier is then digitized using a digitizer.
Test equipment with a high-frequency digitizer is used to achieve an adequate sampling rate of the output sinusoidal waveform. Even with a high-frequency digitizer, the sampling rate is often slower than the data rate of the amplifier. Therefore, the undersampled data are post-processed to reconstruct the waveform. However, digitizing the output waveform in a high-frequency domain and post-processing the data can significantly increase the test time.
Testing a device that has a large gain can be challenging. For example, equalizers used for high-speed non-return-to-zero (NRZ) signals are typically composed of an analog equalizer block with a 20˜30 decibel (dB) gain and a limiting amplifier block with 30˜40 dB gain that amplifies the output signal of the equalizer. In order to measure the linear gain of the analog equalizer block with the extra gain of the limiting amplifier block, the test input signal is extremely attenuated so that the output signal of the limiting amplifier is still linear and not limited.
However, attenuating the test input signal can increase the test time, because the attenuated test input signal may have a poor signal-to-noise ratio with respect to the noise floor of the test equipment. The measured data can be averaged to maintain accuracy, but the averaging process increases the test time even more.